This invention relates to venturis, i.e. tubes through which a fluid is passed having flaring ends connected by a constricted middle and in particular to an easily manufactured venturi of unusual geometry and methods and apparatus for forming the venturi.
Venturis can be beneficially employed in mufflers to attenuate engine exhaust noise. See, for example, U.S. Pat. Nos. 4,267,899 to Wagner et al, 3,672,464 to Rowley et al and commonly assigned 4,361,206 to Tsai. A significant characteristic militating against the widespread use of venturis in mufflers is the cost associated with their manufacture. As can be seen from the aforesaid patents, the venturis heretofore employed in muffler construction have been concentrically symmetric having substantially circular transverse cross-sectional areas along their entire area. Formation of such conical sections is a complex manufacturing problem. For example, the metal venturis used in mufflers are typically fabricated from flat sheet metal stock. One approach used to form venturis from such sheet metal stock has been to fabricate cylindrical, frustro-conical and/or frustro-conically top cylindrical sections from flat sheet stock by cutting, bending and joining sides of the cut stock, typically by welding or brazing, and then joining two or more of the sections, again typically by welding or brazing, to form the venturi. The number of separate cutting, bending and joining steps required adds greatly to the cost of this method of manufacture. Alternatively, such venturi shapes have been formed from cylindrical tubes of substantially constant cross-sectional diameter by flaring the ends or inwardly spinning the center of the tube or both as is described, for example, in U.S. Pat. Nos. 3,120,206 and 2,892,253. The former operation can easily result in splitting of the tube if not properly performed. The latter operation can result in crimping of the sidewall. Both processes become more difficult to perform where the side wall of the tube is seamed. Moreover, several successive flaring or spinning operations or both may be necessary to form an abrupt constriction in the tube without damage.